Automotive manufacturers and designers have long understood the relationship between spark plug gap ionization current and engine misfire. Specifically, it is known that the electrical conductivity within a spark plug gap increases following successful ignition due to the ionization of hot combustion gases. Thus, if a voltage on the order of 90 to 400 volts is impressed in the gap following ignition and the ionization current is measured, a high current flow is known to indicate combustion. A low or zero current flow is similarly known to indicate a misfire condition.
To generate the ionization current sought to be measured, prior art detection circuits have utilized an additional power source coupled to the primary and secondary windings of the vehicle ignition system coils. As those skilled in the art will recognize, however, such designs have generally proven unreliable and thus impractical for on-board detection system use because of the presence of high negative voltages which are inherently produced as a result of the spark event. To address this inherent problem, prior art detection systems have further incorporated complicated signal processing components. See, for example, U.S. Pat. No. 4,547,734 issued to Spaude, which discloses a misfire detection system which utilizes capacitance coupling to the spark plug leads. Spaude also discloses high pass filtering which is inherently susceptible to system noise to detect sharp voltage transitions resulting from ionization breakdown. These components, such as disclosed in Spaude and other prior art detection circuits and methods, unfortunately have only slightly improved the already imprecise method of ionization current measurement while adding substantial cost and complexity to the systems on the whole.